Well, it's *possible*, though not likely, that the 117s are just on the edge of what the twist rate of the barrel's rifling can stabilize, and the 120s are starting to cross the line, giving poor accuracy. Not so much from the 3 grains of weight difference, but because the flat-based bullet has more bearing surface on the rifling, compared to the boattail design; this fact combined with the weight being toward the upper end of the spectrum for bullet weights for standard rifling twist rates. This is mostly speculation, but it seems an educated guess.